1. Field of the Invention
The present invention relates to voltage monitoring devices applied to battery packs having a plurality of unit cells connected in series, in particular, flying capacitor type voltage monitoring devices capable of monitoring and detecting a voltage of each of unit cells forming a battery pack by using a plurality of capacitors.
2. Description of the Related Art
There have been known and used a flying capacitor type voltage monitoring devices capable of monitoring a voltage of unit cells forming a battery pack by using capacitors. The unit cells are connected in series together in the battery pack. Such a flying capacitor type voltage monitoring device has a voltage detection circuit capable of detecting a voltage of each unit cell in the battery pack on the basis of a charged voltage of the capacitor. The voltage detection circuit is connected to the capacitor, and detects whether or not a line disconnection occurs.
There has been proposed a double flying capacitor type voltage monitoring device, which is different in structure from the flying capacitor type voltage monitoring device previously described. The double flying capacitor type voltage monitoring device detects a voltage of each of unit cells by using two capacitors, where the unit cells belong to two battery groups. In the structure of the double flying capacitor type voltage monitoring device, respective first voltage detection circuits are connected to the two capacitors, in order to detect a charged voltage of each of the two battery groups.
There is a parent document 1, Japanese patent laid open publication No. JP 2014-77656, which discloses such a double flying capacitor type voltage monitoring device equipped with a first voltage detection circuit and a second voltage detection device. The second voltage detection device is connected commonly to the two capacitors. In this structure, the first voltage detection circuit detects a charged voltage of each of the two capacitors, and the second voltage detection circuit detects a total charged voltage of the two capacitors. This structure reduces the number of switches (i.e. detection lines) through which the capacitors are connected to both ends of the unit cell group having the unit cells connected in series. As a result, the patent document 1 provides the voltage monitoring device having a simple structure.
However, the voltage monitoring circuit disclosed in the patent document 1 performs both a first process and a second process. The first process charges each of the capacitors, respectively. The second process collectively charges the capacitors connected in series. This introduces a possible problem of performing the second process due to a state in which the charged voltage of each of the capacitors connected in series has a different polarity due to the execution of the first process. This causes a possible problem of generating variation in the charged voltage between the capacitors.
There is another possible problem in which some of the capacitors have a charged voltage which is higher than that of the remaining capacitors due to the variation of the charged voltage between the capacitors. In order to avoid this problem, it is necessary for each of the capacitors to have a withstand voltage higher than the voltage of the unit cell. This increases the overall size of the flying capacitor type voltage monitoring device.